Ellipsometry is a nondestructive technique for studying the physical properties of a sample based on changes induced in the polarization state of a light beam interacting with the sample. In all ellipsometer systems, a light beam having a known polarization state is reflected from or transmitted through the sample. The polarization state of the beam after it interacts with the sample is then measured. The differences in the polarization state of the beam before and after the interaction with the sample can be used to calculate parameters of the sample. In the subject application, the term polarization state is intended to mean the respective amplitudes and phase difference between the p and s polarization components.
The most common form of ellipsometry is reflection ellipsometry where a light beam is directed at an oblique angle of incidence to the sample surface. The polarization state of the reflected beam is measured to derive information about the surface of the material. Common parameters which are studied include the index of refraction and extinction coefficient of the sample. More recently, reflection ellipsometry has been proposed to study carrier densities in a semiconductor wafer (See U.S. Pat. No. 4,472,663, issued Sept. 18, 1984 to Motooka). The most common industry use for ellipsometers is in the characterization of thin film layers on semiconductor samples. Reflection ellipsometry is the best method available for measuring the thickness of very thin layers on semiconductors.
It is also possible to use ellipsometric techniques to analyze a light beam which is transmitted through a sample. This method is referred to as transmission ellipsometry or polarimetry. As in reflection ellipsometry, the polarization state of the incoming beam is compared to the polarization state of the beam after it has passed through the sample. Polarimetry is used to study bulk properties of transparent materials such as the birefringence of crystals. For the sake of simplicity, the discussion in the remainder of this application will be limited to reflection ellipsometry, however the subject invention is applicable to both reflection and transmission ellipsometry.
Turning to FIG. 1, there is illustrated the basic form of a prior art ellipsometer for evaluating the parameters of a sample 10. As shown therein, a means, such as laser 12, generates a beam of radiation 14. This beam is passed through a polarizing section 16 for creating a known polarization state of the beam. The polarization section 16 can include one or more components which will be discussed below. The beam is then reflected off the sample at an oblique angle of incidence .theta. with respect to the normal N as shown in FIG. 1. The reflected beam is then passed through an analyzing section 18 for isolating the polarization state of the reflected beam. The intensity of the beam is then measured by a photodetector 20. A processor 22 can then be used to determine parameters of the sample 10 by comparing the polarization state of the input beam with the polarization state of the reflected beam.
There are a wide variety of ellipsometer schemes which have been developed in the prior art. A thorough discussion of many prior art systems can be found in the comprehensive text by R. M. A. Azzam and N. M. Bashara entitled, Ellipsometry and Polarized Light, North Holland Publishing Co., New York, NY, 1977. In the latter text, significant detail is provided as to the various types of optical components which can be used in the polarization and analyzing sections 16 and 18. Due to the non-directional nature of optical laws, the sequence in which these various optical components are disposed may be interchanged. Typically, the components utilized can include a linear or circular polarizer, a birefringent device often referred to as a compensator, and a linear or circular analyzer. In operation, one or more of these elements are rotated about the azimuth in a manner to vary the polarization state of the beam. Information about sample parameters is derived from the photodetector output as it relates to the azimuthal positions of the various elements.
One of the earliest developed ellipsometric methods is called null ellipsometry. In null ellipsometry, the change in state of polarization which is caused by the sample is compensated by suitable adjustment of the polarizing and analyzing sections such that the reflected light beam is extinguished by the analyzing section. More, specifically, the azimuthal angle of the elements are rotated until a null or minimum level intensity is measured by the detector.
In a photometric ellipsometer, no effort is made to extinguish the light reaching the detector. Rather, the level of light intensity recorded by the detector is measured and compared with the azimuth angles of the components in the polarizing and analyzing sections to derive information about the sample. The processor utilizes mathematical models, including Fresnel equations, to determine the sample parameters. As described in detail in the Azzam and Bashara text, the mathematical models typically include a calculation for the "ellipsometric parameters" .psi. and .delta.. These parameters are related to the relative magnitudes of the p and s polarization states of the reflected beam as well as the phase delay between those two polarizations by the following equations: ##EQU1##
There are many other ellipsometric techniques that are found in the prior art and will be mentioned briefly below. One such technique is modulated ellipsometry wherein small changes in the optical parameters of a surface that are induced by an external field are measured. (See, Azzam, page 265.) Another approach is interferometric ellipsometry. (See, Azzam, page 262 and in U.S. Pat. No. 4,762,414 issued Aug. 9, 1980 to Grego.) In another approach, the reflected probe beam is split into two or more beams and measured by different detectors (See, U.S. Pat. No. 4,585,384, issued Apr. 29, 1986 to Chastang.)
A new approach is disclosed in U.S. Pat. No. 4,725,145, issued Feb. 16, 1988 to Azzam. In the latter patent, the detector is arranged in a manner to simultaneously function as the analyzer section. More specifically, the detector is arranged to be polarization sensitive and has a partially specularly reflecting surface intended to isolate radiation of a certain polarization state. While the approach in Azzam reduces the number of components, the system is still like other prior art devices in that the polarization state of the reflected probe beam must be known. As will be seen below, the subject invention can be used to substantially improve any of the above described ellipsometric methods and apparatus.
As can be appreciated from the above discussion, each of the ellipsometric methods in the prior art require that the incoming beam strike the surface of the sample at an oblique angle of incidence. This has always been performed by directing the entire beam at an oblique angle with respect to the sample surface and having an independent detection system aligned with the reflected beam for capturing, analyzing and measuring the beam. This approach has some serious limitations. First of all, the beam generation and collection components must be accurately aligned. This alignment must be accurate as to both the angle of incidence and reflection of the beam and the azimuthal angles of the incoming beam. The term azimuthal angle in this sense relates to variations in and out of the plane of the paper of FIG. 1, rather than rotation about the direction of travel of the beam which is relevant when discussing the operation of the polarization and analyzing sections. Difficulties with alignment become extremely important in multiple angle of incidence ellipsometers which will be discussed in greater detail below.
As can be appreciated, alignment problems, while raising issues of complexity and cost, can at least be addressed. A more significant problem with the prior art devices that has not yet been solved arises from the need to direct the beam at an oblique angle of incidence which effectively limits spatial resolution. As used herein, resolution is intended to be a measure of the smallest area within which information can be derived. In the manufacturing of semiconductor devices, information about layer thicknesses within extremely small areas is extremely desirable. However, because of the need to direct the beam at an oblique angle of incidence, it is impossible to tightly focus the beam using high numerical aperture optics. Significant efforts have been made to improve the resolution of ellipsometers, but to date, spot sizes below 25 microns cannot be reliably achieved. As will be discussed below, the subject invention overcomes this problem and permits measurement of spot sizes on the order of 1 micron. Furthermore, the subject approach is self-aligning thereby substantially simplifying the measurement procedure.
There are a number of parameters which are used to define a semiconductor sample having a dielectric layer coated thereon. These parameters include the index of refraction and extinction coefficients of the air, thin-film layer and substrate, as well as the thickness of the thin-film layer. In practice, the extinction coefficients of the air and thin-film layer are negligible. However, this still leaves five sample parameters which can be unknown. In the ellipsometric devices described above, only two quantities .psi. and .delta. are measured and therefore only two of the five parameters can be ascertained such that the other three parameters must be known. In order to solve for more of the unknowns, additional independent measurements must be taken.
In the prior art, the need to obtain additional independent measurements has been addressed with multiple angle of incidence devices. (See, Azzam, page 320) In these devices, the angle of incidence of the beam is changed by varying the angular position of the laser generating components. Measurements are then taken at multiple angles of incidence. The multiple independent measurements allow additional unknown sample parameters to be calculated. Alternatively, the additional independent measurements can be used to calculate the unknown parameters with greater accuracy.
One of the drawbacks of multiple angle of incidence devices is that in order to measure the reflected beam, the angle of the detection components must also be similarly adjusted to capture the reflected beam. As can be appreciated, the need to adjust the position of all of the components makes accurate alignment even more difficult. As will be seen below, another important advantage of the subject invention is that multiple angle of incidence ellipsometry can be performed without adjusting the position of the beam generation or collection components.
Another problem with multiple angle of incidence devices is that they cannot be used to measure relatively shallow angles of incidence. As the thickness of the thin film increases, information from these shallow angles becomes of greater significance. As can be seen in FIG. 1, in order to obtain shallower angles of incidence, the light generating and collecting elements must be rotated up toward each other. The physical size and location of these components will make detection of angles of incidence of less that 20.degree. quite difficult. As will be discussed below, an ellipsometer formed in accordance with the subject invention can measure multiple angles over a wide range, from 70.degree. down to and including 0.degree..
Accordingly, it is an object of the subject invention to provide a new and improved ellipsometer configuration which is suitable for enhancing performance of all existing systems.
It is still another object of the subject invention to provide a new and improved ellipsometer with enhanced resolution.
It is still a further object of the subject invention to provide a new and improved ellipsometer which is self-aligning.
It is still another object of the subject invention to provide a new and improved ellipsometer which can provide multiple angle of incidence measurements without actively changing the angle of incidence the incoming beam.
It is still another object of the subject invention to provide a new and improved ellipsometer which is capable of determining multiple unknown parameters of a sample.
It is still a further object of the subject invention to provide an ellipsometer device coupled with a total power measurement for enhancing the calculation of sample parameters.